Fetal Cardiac Development

Fetal Cardiac Development

• The cardiogenic field is a "U"-shaped area of blood-forming cavities cranial to the neural plate.
• The cardiac tube is formed from the cardiogenic fields.
• Loop formation occurs when the cardiac tube folds.
• The internal division of the cardiac tube forms:
  • Primitive atrium -> left and right atria
  • Primitive ventricle -> left ventricle
  • Bulbus cordis -> right ventricle
  • Truncus arteriosus -> aorta and pulmonary trunk
• Truncus arteriosus is a part of the cardiac tube that divides into the aorta and pulmonary trunk.
• Bulbus cordis is a part of the cardiac tube that becomes the right ventricle.
• Foramen ovale is a passage through the right and left atria.
• Fossa ovalis is the remnant of the foramen ovale after it closes and fuses shut.
• Aortic arches form from the truncus arteriosus through a spiral aorticopulmonary septum.

Fetal Circulation

• The placenta performs gas exchange, not the lungs or liver.
• The umbilical cord has two umbilical arteries and one umbilical vein.
• Deoxygenated blood flows from the right atrium to the left atrium through the foramen ovale.
• Oxygenated blood from the placenta returns to the right atrium through the umbilical vein.
• Blood flow in the fetus:
  • Right atrium -> left atrium -> right ventricle -> pulmonary trunk -> ductus arteriosus -> aorta -> body
  • Right ventricle -> pulmonary trunk -> lungs (minimal flow)

Functional vs. Anatomical Closure

• Functional closure of the foramen ovale: the primary and secondary septum get physically pushed together, closing the gap.
• Anatomical closure of the foramen ovale: the primary and secondary septum fuse together, becoming the fossa ovalis.
• Functional and anatomical closure of the ductus arteriosus occur in a similar manner.

Neonatal Heart Blood Flow

• Blood flow in the neonate:
  • Right atrium -> right ventricle -> lungs -> left atrium -> left ventricle -> body

Vessel Structure and Characteristics

• The three main layers of vessels are:
  • Tunica intima (innermost layer)
  • Tunica media (middle layer)
  • Tunica adventitia (outermost layer)
• Artery types:
  • Elastic arteries (e.g. aorta) have mostly elastic lamellae in the tunica media.
  • Muscular arteries (e.g. femoral artery) have mostly smooth muscle in the tunica media.
• Arterioles have the greatest effect on blood pressure and have a single layer of smooth muscle cells.
• Metarterioles have sphincters that regulate blood flow into capillaries.
• Capillaries:
  • Are one cell layer thick
  • Allow for nutrient and gas exchange
  • Have three types: continuous, fenestrated, and sinusoidal
• Venules:
  • Are also known as postcapillary venules
  • Have no smooth muscle
  • Are very "leaky"
  • Allow for leukocyte diapedesis
• Veins:
  • Have three layers: tunica intima, tunica media, and tunica adventitia
  • Valves are present in veins to prevent backflow
• Lymphatic vessels:
  • Have a surface of mesothelium that secretes serous fluid
  • Have a variable thickness of adipose tissue and coronary arteries and veins

Cardiac Cells and Tissues

• Purkinje cells:
  • Are modified cardiomyocytes that aid in conduction
  • Are also known as fibers
• Intercalated discs:
  • Include gap junctions and desmosomes
  • Allow for communication and strength between cardiomyocytes
• Striated cardiac muscle: composed of cardiomyocytes
• Pericytes:
  • Are contractile cells that wrap around endothelium in capillaries and venules
  • Are important for homeostasis and angiogenesis
• Podocytes:
  • Are located in nephrons
  • Help prevent large molecules from being filtered by increasing surface area with pedicals

Types of Capillaries

• Continuous Capillaries: most common type of capillary, found in brain, bone, and lungs
• Fenestrated Capillaries: allow fluid exchange, found in intestinal villi, choroid plexus, and glomerular capillaries (renal)
• Sinusoid Capillaries:
  • Lumen is enlarged and irregular
  • Endothelium is discontinuous and fenestrated
  • Basal lamina is discontinuous
  • Larger molecules are able to exit/enter
  • Found in spleen and liver

Pericytes

• Also known as Rouget cells
• Contractile cells that wrap around the endothelium of capillaries and venules
• Communicate through physical contact and paracrine signaling

Functions of Pericytes

• Serve as a stem cell source
• Proliferate after injury to aid in repair
• Vital for angiogenesis, enabling the formation of new blood vessels
• Play a crucial role in maintaining homeostasis

Discover the stages of fetal cardiac development, from the formation of the cardiac tube to the internal division of the heart chambers.